Present-day Internet communications represent the synthesis of technical developments begun in the 1960s. During that time period, the Defense Department developed a communication system to support communication between different United States military computer networks, and later a similar system was used to support communication between different research computer networks at United States universities.
The Internet
The Internet, like so many other high tech developments, grew from research originally performed by the United States Department of Defense. In the 1960s, Defense Department officials wanted to connect different types of military computer networks. These different computer networks could not communicate with each other because they used different types of operating systems or networking protocols.
While the Defense Department officials wanted a system that would permit communication between these different computer networks, they realized that a centralized interface system would be vulnerable to missile attack and sabotage. To avoid this vulnerability, the Defense Department required that the interface system be decentralized with no vulnerable failure points.
The Defense Department developed an interface protocol for communication between these different network computers. A few years later, the National Science Foundation (NSF) wanted to connect different types of network computers located at research institutions across the country. The NSF adopted the Defense Department's interface protocol for communication between the research computer networks. Ultimately, this combination of research computer networks would form the foundation of today's Internet.
Internet Protocols
The Defense Department's interface protocol was called the Internet Protocol (IP) standard. The IP standard now supports communication between computers and networks on the Internet. The IP standard identifies the types of services to be provided to users and specifies the mechanisms needed to support these services. The IP standard also describes the upper and lower system interfaces, defines the services to be provided on these interfaces, and outlines the execution environment for services needed in this system.
A transmission protocol, called the Transmission Control Protocol (TCP), was developed to provide connection-oriented, end-to-end data transmission between packet-switched computer networks. The combination of TCP with IP (TCP/IP) forms a system or suite of protocols for data transfer and communication between computers on the Internet. The TCP/IP standard has become mandatory for use in all packet switching networks that connect or have the potential for utilizing connectivity across network or sub-network boundaries.
A computer operating on a network is assigned a unique physical address under the TCP/IP protocols. This is called an IP address. The IP address can include: (1) a network ID and number identifying a network, (2) a sub-network ID number identifying a substructure on the network, and (3) a host ID number identifying a particular computer on the sub-network. A header data field in the information packet will include source and destination addresses. The IP addressing scheme imposes a sensible addressing scheme that reflects the internal organization of the network or sub-network. All information packets transmitted over the Internet will have a set of IP header fields containing this IP address.
A router is located on one or more networks, and routers are used to regulate the transmission of information packets into and out of computer networks and within sub-networks. Routers are referred to by a number of names including Home Agent, Home Mobility Manager, Home Location Register, Foreign Agent, Serving Mobility Manager, Visited Location Register, and Visiting Serving Entity. A router interprets the logical address of an information packet and directs the information packet to its intended destination.
The TCP/IP network includes protocols that define how routers will determine the transmittal path for data through the network. Routing decisions are based upon information in the IP header and entries maintained in a routing table. A routing table possesses information for a router to determine whether to accept the communicated information packet on behalf of a destination computer or pass the information packet onto another router in the network or sub-network. The routing table's address data enables the router to accurately forward the information packets.
The routing table can be configured manually with routing table entries or with a dynamic routing protocol. In a dynamic routing protocol, routers update routing information with periodic information packet transmissions to other routers on the network. This is referred to as advertising. The dynamic routing protocol accommodates changing network topologies, such as the network architecture, network structure, layout of routers, and interconnection between hosts and routers.
The IP-Based Mobility System
The Internet protocols were originally developed with an assumption that Internet users would be connected to a single, fixed network. With the advent of portable computers and cellular wireless communication systems, the movement of Internet users within a network and across network boundaries has become common. Because of this highly mobile Internet usage, the implicit design assumption of the Internet protocols has been violated.
In an IP-based mobile communication system, the mobile communication device (e.g. cellular phone, pager, computer, etc.) is called User Equipment. User Equipment is sometimes called a mobile node, mobile unit, mobile terminal, mobile device, or similar names depending on the nomenclature adopted by particular system providers. User equipment could also include fixed equipment that does not move from its home network, such as a desktop computer.
User Equipment may change its point of attachment to a foreign network while maintaining connectivity to its home network. User Equipment may also change its point of attachment between sub-networks in its home network or foreign network. The mobile node will always be associated with its home network and sub-network for IP addressing purposes and will have information routed to it by routers located on the home and foreign network. Generally, there is also usually a correspondence node, which may be mobile or fixed, communicating with the mobile node.
IP Mobility and Care-Of Addressing Protocols
In a mobile IP network, nodes will transmit notification and discovery information packets onto the network to advertise their presence on the network and solicit advertisements from other nodes. While on a foreign network, User Equipment will be assigned a care-of address that will be used to route information packets to the foreign network and the attached mobile node. An advertisement from a router on the foreign network will inform User Equipment that is attached to a foreign network.
The mobile node will typically create a care-of address on the foreign network, which it will transmit to its home network in an information packet to register the care-of address. Information packets addressed to the mobile node on the home network have the care-of address added. This information packet containing the care-of address will then be forwarded and routed to the mobile node on the foreign network by a router on the foreign network according to the care-of address.
Application of Policy in Multimedia Networks
Policy is used to control how different types of IP traffic are treated within multimedia networks. This may include the application of particular charging rules, or the quality of service provided to the given traffic by the network routers. Quality of service includes the blocking of unauthorized traffic, and if incorrect policies are applied to traffic, the quality of service rules may dictate blocking of traffic instead of allowing the traffic to flow.
In order for the correct policy to be applied to a given traffic flow, the IP packets belonging to that flow must be properly recognized. This process is called classification. Classification uses a description of the traffic called a classifier to assist with this process. A classifier is also known as a traffic filter or flow description. A classifier typically contains such attributes as Source IP Address, Destination IP Address, Source IP Port, Destination IP Port, and Protocol, and may contain additional attributes.
Classification is performed at a policy enforcement function. In order for the policy enforcement process to be successful, the addresses and ports used to construct the classifier must match the addresses and ports that will be used to route the IP packets as they appear at the policy enforcement function. IP packets received by the policy enforcement function which have IP routing headers containing fields which match all of the relevant fields of a given classifier are classified correctly and the proper rules are applied to those packets.
The application of policy in IP multimedia networks is generally distributed between an Application Function (AF), a Policy and Charging Rules Function (PCRF) and a Policy and Charging Enforcement Function (PCEF). The Application Function (AF) is directly involved in the establishment of multimedia sessions. A Policy and Charging Rules Function (PCRF) receives information about session establishment from the AF and applies operator defined business policy. And, a Policy and Charging Enforcement Function (PCEF) is directly involved in the media path and is ultimately responsible for enforcing policy and charging rules.
Examples of an AF include voice over IP soft-switches, IMS call session control function (CSCF) or application server (AS). A PCRF is also known as a policy decision function (PDF), policy decision point (PDP) or simply as a policy function (PF). A PCEF is also known as a policy enforcement point (PEP) or policy enforcement function (PEF). Each of these devices require accurate address information to operate on the distributed policy architecture.
Network Address Translation
Network address translation devices, sometimes called network address and port translation devices, are often integrated into session border controller devices or application layer gateway devices. These session border controller devices are located on the border of a private network and a public network, and the network address translation devices function by replacing the local source address for the mobile node sending an outgoing packet with the public source IP address for the public network. A reverse translation occurs when a packet meant for transmission to the mobile node is received by the network address translation device or the session border controller device.
The network supporting communications to the mobile node (and having a network address translators and session border controller devices) may also possess devices that control rules, charging, and policies established for transmissions associated with the mobile node. There are control rules, charging and policy devices, often called the Policy Control and Charging Rules Function (PCRF) and the Policy Control and Charging Enforcement Function (PCEF), that control and support these rules and policies on the network. The rules and policies associated with User Equipment are applied based on the mobile node's source address. Difficulties arise when these other devices on the network are asked to apply rules and policies based on translated addresses, as opposed to the mobile node's actual source address. When the PCEF does not receive communication of addressing translations, there is a need for coordinated information to assist in the uniform policy enforcement and packet forwarding of transmissions.
For instance, multimedia communications may be requested by User Equipment. These multimedia communications are coordinated with a device called the Proxy Call Session Control Function (PCSCF) located on a separate network that supports the service providers for the network multimedia content providers. The PCSCF is often provided with a translated address that has been modified by the network and port translation device or the session border controller device during the session set-up.
The PCSCF must use translated address information to communicate with PCRF and PCEF functions, but these devices may only possess the mobile node's actual source address—not the translated address provided to the PCSCF. The translated address for the mobile node has not been effectively communicated to these other devices on the network. The lack of coordination and communication of translated address information in the session set-up creates several problems, such as the following: (1) media packets being routed to incorrect destinations, (2) policy requests being routed to incorrect policy enforcement devices (PCEF), (3) mismatched addresses and ports that are not recognized by traffic filters and classifiers applied in the policy enforcement devices (PCEF), and (4) lack of corresponding flow descriptions where policy rendezvous is used. There is a need to more accurately coordinate the correct application of policy in networks where network address translation and network address and port translation functions are present.